Research On Dynamic Virtual Impedance Optimized Parallel Scheme For Distributed VSC

Paralleled multi-inverter technology is one of the critical technologies to distributed generation system(DGS). A large number of distributed resources will be connected with AC bus by using inverters. Among different kinds of parallel operation mode, voltage source converter(VSC) can not only supply power, but also maintain the magnitude and frequency of voltage. The system is more redundant. Consequently, this thesis represents an optimized scheme based on droop algorithm, which has advantages of automatic load sharing without interconnections, working smoothly all through the grid-connected and islanding situations. The main works are as follow:First of all, in order to satify the impedance characteristic requirment of low voltage grid lines, it is necessary to change the control relationships of traditional droop algorithm according to the active power-magnitude and passive-frequency decoupling control method. Control system frame structure with double loops has been desigened to tail after the droop voltage. The instantaneity and accuracy of different current sampling ways also have been considered. Meanwhile the virtual impedance analysis method is added to modified output impedance characteristic and the forward feedback scheme is adopted to compensate the virtual voltage drop.Secondly, two innovated optimized schemes are put forward. To minimize the circulating current and prevent inverter excessive output voltage drop, a dynamic virtual impedance adjusting technology is deduced from the static optimal parameter design by modify the feedback coefficent on line. The other improvement is to add intergal and differential links to droop algorithm on the premise of the system stability, with the purpose of both recovering the magnitude and frequency cause by orginal droop calculation and enhancing the dynamic performance during the load sharing process.Thirdly, A parallel system with two inverters has been constructed on the Matlab/Simulink platform to verificate the feasibility of the algorithm theory under simulative low power line environment. Comparison between innovated optimized scheme and traditional control technology has been carried out to indicate the better load sharing ability of the former method.At last, through design work of software and hardware system platform, the optimized parallel algorithm is applied on TMS320F28335. Different module and output performance of two single phase inverters is tested. Parallel system experiments on the platform with single capacity of 1kVA. The results and analysis demonstrate the correctness and validity of this scheme. The contents of this item has a theoretical and practical significance in the further research and application of the distributed generation system.